Corrosion is a serious concern in the maritime environment. A mix of salt, water and air is about the worst possible environment for metal objects to survive in, and if corrosion isn't dealt with, usually at significant expense, it rapidly becomes a safety issue.

Marine corrosion is electrochemical in nature. For various reasons, different portions of a metal will have different electrical potential, causing electrons to flow between them. Areas with extra electrons (known as cathodic areas) give them up to the sea, combining with water and dissolved oxygen to produce hydroxyl ions (OH-). These are then drawn back to the areas which gave up their electrons, known as anodes, where they combine with the atoms of metal to form metal oxides. If the ship is primarily made of iron or steel, the product is hydrated iron oxide, more commonly known as rust. Rust is generally quite fragile, and doesn't protect the underlying surface very well. Other metals, such as aluminum and stainless steel, also corrode, but the products of said corrosion instead cling tightly to the surface, protecting the underlying metal from further corrosion. In fact, you may know aluminum rust by another name, sapphire. Read more...
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